A closed-caption system aids the heating-impaired in enjoying television programs. In such a system, text corresponding to the words spoken in a television program are transmitted with the picture and sound information from the broadcast transmitter. The closed-caption text is displayed at the bottom of the screen (similar to the way in which motion picture subtitles are displayed) so that a hearing-impaired viewer may better understand the television program.
The Television Decoder Circuitry Act of 1990 requires the inclusion of a closed-caption decoder in a television receivers sold in the United States having display screens which are thirteen inches (approximately 33 cm.) or larger (measured diagonally). In a closed-caption system implemented in accordance with this Act, closed-caption text is transmitted a few characters at a time during the vertical blanking interval on television line 21. The above-mentioned closed-caption decoder captures the data on television line 21, and displays it via on-screen display circuitry. It should be noted that at the present time not all television programs are closed-captioned.
The FCC has mandated that the closed-caption display circuitry must be capable of being turned on or off at the viewers option. Clearly, the vast majority of purchasers of new television receivers are not hearing-impaired, and have little or no need for the closed-captioning capability built in to their new television receivers. Therefore, it is felt that the default state of the closed-captioning system should be the "off" state.
Unfortunately, if the default state of the closed-captioning system is the "off" state, a problem arises in that the viewer will not know when closed caption data is available for display, because, as noted above, not all television programs are accompanied by closed-caption data.
In FM stereo receivers, it is common practice to light a stereo-available indicator in response to the detection of the stereo pilot signal. Detection of the stereo pilot signal is a reliable indicator of the presence of stereo audio because the pilot signal always accompanies a stereo broadcast. At first glance, it might appear that one should look for a similar signal which always accompanies closed-caption information, such as the closed-caption run-in clock, or look for a NEW-CHARACTER-RECEIVED (i.e., NEWDATA) signal from the closed-caption data slicer. Unfortunately, unlike the stereo pilot signal of the above-mentioned FM stereo example, the run in clock may be present even in the absence of closed-captioning because many television broadcasters do not turn off their closed-caption encoders in the absence of closed-caption information, but rather send a series of null characters (e.g., the code "00") on line 21. Moreover, in the absence of closed-caption information, the encoders of some broadcasters transmit a sequence of closed-caption commands which cause an erasure of data which may be remaining on the screen if, for example, a user changed from a channel which was transmitting a closed-captioned program to a channel which was not. Thus, even though no useful information is being sent, the run-in clock is present and actual closed-caption characters (i.e, null characters, or an erase sequence) are being received. For these reasons, neither the run-in clock nor a NEW-CHARACTER-RECEIVED signal is a reliable indicator of the presence of useful closed-captioning.